Gary Nichols wrote:
> With the new receive system the tower is destroying the pattern especially
> when looking
> NE or SE but it still seems to hear pretty well in the NW and SW directions
> looking away from the tower.
>
>
>
> After talking to Goose, W8AV I am following his advice to install a 3 drop
> wire de-tuning system to eliminate the interaction between the tower and RX
> system. My question for the group is: If I choose to use the de-tuning
> skirt for a transmit feed system instead of the short gamma rod system can I
> expect to see any improvement in my transmit signal?
Gary,
The advantage of a three or four wire cage is greater bandwidth, a less
reactive feed, and as Goose mentioned the ability to open up the feed
point to the bottom of the cage and have it act as an anti-resonant
decoupling sleeve in respect to the tower. This all depends on the cage
size, tap point, and how close to a TB resonant frequency the tower is
with or without the cage, open, grounded, or fed via a matching unit. I
think it would be better to model the various cage connections.
Depending on the dimensions, opening up a cage for 160 could give you an
1/2 wave coaxial for 80 meters that could be a instant creation of a
major noise source on 80 meters. It could also make a fine TX antenna
for 80 with another shunt feed above the cage but in theory give you
about 3.2 db over the 1/4 wave and lower the TAO lobe about 10 degrees
which could be good or bad. Yet I have read so many accounts that claim
a 1/2 wave vertical on TB and 80 are not much of a benefit.
To try and answer your question, no, there is not any noticeable
efficiency improvement with a cage feed according to the the NAB
presentations I have read. There is better B/W and an ability to
control re-radiation via the anti-resonant decoupling sleeve and feed
concept. How much? I'm not sure and don't think the modeling will be
much help there except to reflect the highest reactance which I presume
equates to less re-radiation.
But also remember that even steel tower guys that are broken up at 1/8
wave intervals can contribute some noise to an exotic RX system close
by. There are a number of IEEE and NAB presentations on detuning large
metal power line supports in the vicinity of multi-tower directional AM
stations that are required to prove there patterns to the FCC.
73
Herb Schoenbohm, KV4FZ
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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